One of the applications of encased cylindrical filters is filtering lubricating oil in combustion motors so as to keep the oil free of impurities. A second typical field of application for filters of this type is filtering hydraulic liquid in hydraulic systems. Since the filter is fouled in operation and has to be replaced at intervals, the filter casing must be removable from the device assembly, to which it is attached.
There are previously known oil filters, in which the filter means and the surrounding casing have been constructed as a solid unit, and are replaceable. However, such a design is a waste of material and also produces landfill waste.
From an ecological viewpoint, it is preferable to devise a casing that can be opened, so that the clogged filtering means alone is replaced and the casing is put back in position with the device assembly. U.S. Pat. No. 5,342,519 is cited as an example of such a design, in which the assembled oil filter comprises a cylindrical, replaceable filter means, a perforated support pipe within the filter means, a collar as an extension of the support pipe, a casing surrounding the filter means and a joint ring at the casing opening. This filter design includes two threaded joints with O-ring seals, one of which connects the casing to the joint ring and the other which connects the assembled filter with the collar to the device assembly so as to provide a seal between the joint ring and the device assembly. There are also conventional operable filter designs in which one single threaded joint is sufficient to connect a unit consisting of a filter means, a casing and any joint members to the device assembly.
In replaceable filters, the casing must be tightly sealed to the device assembly, since otherwise the medium to be filtered, such as pressurized lubricating oil or hydraulic liquid, will extrude through the joints between the elements into the environment. Such a visible leak will immediately reveal the deficiency or absence of a seal. In contrast, current filter designs do not show outwardly whether the filter means is fitted in position or not. With such filters, it is quite possible to remove a clogged filter means and subsequently put the casing back in position, without replacing the removed filter means. Obviously, as a result of such a mistake filtering will no longer take place.